There are two periods in the life of a truck wheel bearing that small metallic particles are most likely to appear in the lubricating oil of truck wheel bearings. The first period is when the wheel bearing is new and the parts are seating. It is desirable to remove these particles from the fluid as they appear and thereby prolong the life on the bearings. The second period is when the bearing is nearing the end of its life and the bearing members begin to wear excessively. The presence of unusual amounts of metallic particles in the lubricating oil can signal the need to change wheel bearings.
A truck wheel bearing hub encloses a chamber that is roughly one fourth filled with lubricating oil. An access plug is centered in the hub cap assembly on the axis of revolution of the wheel and above the oil level. The wheel bearing is lubricated by the splashing of oil on the bearings due to the rotation of the wheel when the truck is moving.